How to Calculate Transformation Efficiency (CFU/µg)

After you transform competent cells and plate them, the raw colony count means almost nothing on its own: 200 colonies could be an excellent result or a dismal one depending on how much DNA you used and how much of the culture you actually spread. Transformation efficiency normalizes all of that into a single comparable number — colony-forming units per microgram of DNA (CFU/µg) — so you can judge your competent cells and your DNA against the expected range. This guide derives the formula, walks the plated-volume chain through four real examples, and explains what a low value tells you. The transformation efficiency calculator runs the same arithmetic and flags which competency range your number lands in.

What transformation efficiency measures

Transformation efficiency answers one question: how many transformants did you recover per microgram of DNA? A transformant is a cell that took up the plasmid and grew into a colony, so one colony is taken to represent one colony-forming unit (CFU). Dividing the number of CFU by the mass of DNA that produced them gives CFU/µg, a figure that is directly comparable across experiments, cell types and labs — provided the volume chain is handled correctly.

It measures the transformation step only. It says nothing about whether the colonies carry the right construct; a high efficiency with the wrong insert is still a failed clone. For that you need colony PCR or a digest downstream.

The formula and the plated-volume chain

You almost never plate the whole transformation. You plate an aliquot of a larger recovery volume, so the colonies on the plate represent only that fraction of the total transformants. Two corrections follow from this:

dilution factor = recovery volume / plated volume

total DNA (µg) = DNA (ng) / 1000

plated DNA (µg) = total DNA × (plated volume / recovery volume)

TE (CFU/µg) = colonies / plated DNA

The same plated-volume fraction does two jobs in opposite directions: it scales the colonies up to the whole culture and scales the DNA down to what was on the plate. Because the two uses are easy to confuse, keep them as separate visible steps rather than collapsing them into one number. This is exactly the plate-count correction used by the CFU calculator for viable-cell concentration, just normalized to DNA mass instead of sample volume.

Worked example: a routine chemical transformation

200 colonies from plating 100 µL of a 1000 µL recovery, after transforming with 1 ng of plasmid.

dilution factor = 1000 / 100 = 10
total DNA = 1 ng / 1000 = 0.001 µg
plated DNA = 0.001 × (100 / 1000) = 1 × 10⁻⁴ µg
TE = 200 / 1 × 10⁻⁴ = 2 × 10⁶ CFU/µg

That is a typical chemically-competent result. Equivalently, TE = colonies × dilution factor ÷ total µg = 200 × 10 / 0.001 = 2 × 10⁶ — the same answer by the shortcut form.

Reading the result against expected ranges

Cell type	Typical efficiency (CFU/µg)
Chemically competent	10⁶ – 10⁸
Electrocompetent	10⁹ – 10¹⁰ or higher

A value far below the expected range for your cell type is the useful signal. Work backwards through the usual culprits: were the cells kept cold and used within their window, was the heat shock or electroporation pulse correct, was the DNA clean and supercoiled, was the antibiotic plate fresh? Salt or ethanol in the DNA is a classic cause of a tenfold drop — it makes electroporation samples arc and chemically-competent cells transform poorly.

Worked example: electroporation with picogram DNA

Electroporation is so efficient that you use far less DNA. 500 colonies from 100 µL of a 1000 µL recovery, transformed with 0.001 ng (1 pg):

dilution factor = 1000 / 100 = 10
total DNA = 0.001 ng / 1000 = 1 × 10⁻⁶ µg
plated DNA = 1 × 10⁻⁶ × (100 / 1000) = 1 × 10⁻⁷ µg
TE = 500 / 1 × 10⁻⁷ = 5 × 10⁹ CFU/µg

Five billion CFU/µg sits squarely in the electrocompetent range — a number that would be impossible to reach chemically, and the reason libraries are built by electroporation.

Worked example: plating the whole recovery

Sometimes you plate everything, e.g. a small recovery volume spread in full. 300 colonies, 1000 µL plated of 1000 µL recovery, 2 ng DNA:

dilution factor = 1000 / 1000 = 1 (the whole culture was plated)
total DNA = 2 / 1000 = 0.002 µg; plated DNA = 0.002 µg
TE = 300 / 0.002 = 1.5 × 10⁵ CFU/µg

When the plated volume equals the recovery volume the dilution factor is 1 and the plated DNA equals the total DNA — a useful sanity check that you entered the volumes the right way round.

Countable range matters

The arithmetic is only as good as the colony count. Use a plate in the countable range (roughly 20–300 colonies). A confluent lawn cannot be counted at all, and a plate with three colonies gives a wildly noisy estimate — both make the efficiency meaningless no matter how correct the math is. If your plate is out of range, transform again with less DNA, or plate a smaller aliquot, and use the CFU calculator to handle replicate plates and the countable-range warning. Cell clumping or uneven spreading biases the count in the same way: one clump gives one colony but represents several cells.

DNA mass, form and size all move the number

Efficiency depends on the DNA itself, so compare like with like:

Mass entered. Enter the mass you actually added to the cells in ng, not the stock concentration. If you measured the plasmid by absorbance, the DNA concentration calculator gives you the mass; the plasmid molecular weight calculator tells you how many pmol that was.
Form. Intact supercoiled control plasmid transforms far better than a ligation mixture, which contains nicked, linear and empty-vector species. Never benchmark a ligation against supercoiled control DNA and expect the same efficiency.
Size. Larger plasmids transform less efficiently, roughly inversely with size, so a 12 kb construct gives a lower CFU/µg than a 3 kb one even with the same cells.

Always transform a known control plasmid alongside a new batch of competent cells to separate “bad cells” from “bad DNA”.

Common mistakes

Forgetting the plated-volume dilution factor and reporting colonies per plate as if it were the whole culture — the single most common error, and a tenfold under-report when you plate 100 µL of 1 mL.
Entering DNA in µg when the field expects ng (or vice versa), shifting the answer by 1000×.
Counting a lawn or a near-empty plate and treating it as quantitative.
Using the stock concentration instead of the mass added to the cells.
Comparing a ligation directly against supercoiled control DNA and concluding the cells failed.
Swapping plated and recovery volumes, giving a dilution factor below 1 when it should be above it.

Where this fits in the cloning workflow

Transformation efficiency is the read-out at the end of the cloning chain. You size and quantify your fragments, plan the insert mass with the ligation ratio calculator (the how to calculate ligation ratio guide covers the molar-ratio conversion), ligate, transform, plate, and then compute CFU/µg to see how it went. A low efficiency on a well-planned ligation points at the cells or the DNA; a normal efficiency with no correct clones points at the ligation. All three tools live in the molecular cloning hub and share the same DNA masses, so the numbers stay consistent from sizing through plating.